1. Field of the Invention
The present invention relates to a plasma process apparatus used for a semiconductor manufacturing method such as a sputtering method, ashing method, CVD method, etching method, etc.
2. Description of the Related Art
A plasma process apparatus is constructed so that vacuum discharge is caused to generate plasma in a process container which contains a process gas, and an object to be processed is subjected to specified processes, such as film forming, ashing, etching, etc., by utilizing the plasma.
Conventionally known is a plasma process apparatus which is provided with parallel plate electrodes, for example. This apparatus comprises a process container in which a decompressed space is formed by evacuation, a lower electrode located in the lower part of the process container and holding a semiconductor wafer as an object to be processed, an upper electrode opposed to the lower electrode, and a high-frequency power source for applying a high-frequency voltage between the electrodes to generate plasma therebetween. A gas inlet port for receiving a process gas is formed in the upper surface of the upper electrode, while a number of gas supply holes, through which the received gas is fed into the process container, are formed dispersedly in the lower surface of the upper electrode. According to this arrangement, the gas for plasma process is fed through these gas supply holes into the process container, and after the process, the gas is discharged through an exhaust port from the container.
In subjecting the semiconductor wafer to plasma process by using the plasma process apparatus described above, the plasma process gas is fed through the gas supply ports into the process container, and the high-frequency voltage is applied between the upper and lower electrodes to generate the plasma by electrical discharge between them. The specified plasma process is carried out for the semiconductor wafer on the lower electrode with use of active seeds of the plasma.
In the case of the conventional plasma process apparatus having the parallel plate electrode structure described above, however, the plasma is generated by applying the high-frequency voltage to the upper and lower electrodes in the process container to cause the electrical discharge between the electrodes. Accordingly, the discharge gas pressure is restricted by the relationships between the discharge starting voltage, inter electrode distance, and gas pressure. A gas pressure of about 0.5 Torr is the upper limit of the degree of vacuum for stable generation of the plasma between the upper and lower electrodes, and the plasma cannot be generated in a higher vacuum. If the plasma process is effected under the gas pressure of this level, active seeds, such as ions, in the plasma run against the electrodes, thereby spattering thereon, so that impurities are generated from the electrodes. Thus, the semiconductor wafer is soiled by the impurities, and therefore, the yield is lowered. With the recent progress of superfine working technique, moreover, semiconductor wafers have come to require working in a high vacuum. However, the conventional plasma process apparatus cannot meet this requirement.